The invention generally relates generally to synchronization pulse detection and more particularly to detection of synchronization pulses used in video signals.
As is known in the art, it is frequently required to provide a synchronization pulse prior to the active video information portion of a signal. For example, with video signal such as that shown in FIG. 1, each time a line, or segment, of video information 25 is provided, such line, or segment, is preceded with a horizontal synchronization pulse (i.e., Hsync). Thus, as shown in FIGS. 1 and 1A, each Hsync has a substantially non-time varying tip portion disposed between a pair of substantially time varying transition portions. The Hsync is preceded by a "front porch" and is terminated by a "back porch". The "color burst" signal resides within the "back porch". The "front porch" and "back porch" porch signals are at the "blanking level". The active video information 25 is provided between the termination of the "back porch" and the "front porch" of the next Hsync.
Horizontal synchronization pulses (Hsyncs) in video signals are used to identify the begin of a line, or segment, of video information 25. Hence, the accurate detection of the Hsync is crucial to the correct processing of the contents of a horizontal line, or segment, of video information 25. This is true for analogue video as well as its digitized equivalent. In some Hsync detection systems, the 50% point of their falling edge is used as a reference point for the timing of the rest of the line, or segment, of video. In order to detect an Hsync, many detection circuits rely on the fact that the Hsync extends substantially below the "blanking level" for a considerable length of time. In some of these systems, Hsync detection has been carried out by comparing the actual video signal amplitude with a threshold amplitude set below the amplitude of the "blanking level". Various filtering algorithms have been developed to avoid false triggering, for instance triggering on short glitches in the video signal. The threshold amplitude in the simplest implementation is of a DC nature. More advanced developments make the threshold amplitude adaptive to the incoming video signal.
The underlying assumption for all these algorithms is that the "blanking level" is known by the time Hsync detection takes place. This may not necessarily be the case since the transmission of a video signal usually trends to distort, or even lose, the DC value of a video signal. Many pieces of video equipment also rely on AC-coupling to connect video sources to them. As a result, the "blanking level" of the video signal is unknown. In practice, video signal clamp circuits are used to restore the DC value (See, for example U.S. Pat. No. 5,003,564). Only after the clamping process is finished, and the clamping control loop has settled, can one safely assume any DC value (e.g., "blank level", Hsync tip value, etc.) of the video signal to have been restored and therefore be known. Thereafter, the threshold value for Hsync detection can be determined by offsetting from the "blanking level".
While these techniques may be adequate for standard video signals from good quality signal sources under conditions where a fast lock-in time is not of high importance, they are very sensitive to DC shifts within the video signal. Furthermore, the initial lock-in depends on a successful clamp (i.e., successful restoration of the DC value). There also is a time penalty with this technique since the clamping and Hsync detection now become sequential tasks. Improved versions of Hsync detection use adaptive thresholds (See, for example, U.S. Pat. No. 5,576,770) which produce an adaptively restored DC value. While this provides a more robust function of the detection circuit during lock, it still relies on the performance of the clamping circuit (i.e., DC restoration) for initial lock and uses a principle known as "sync slicing", i.e., the comparison of a signal amplitude with the adaptively restored pseudo DC value.